Set 3

Transcript

0:00Introduction

Hello everyone! Okay, one small puzzle before we start. Ammonia has a lone pair on its nitrogen, and in acid, a proton goes straight to that nitrogen. Now take ethanamide, the amide of ethanoic acid. It also has a nitrogen with a lone pair. So in strong acid, the proton goes to the nitrogen again, right? Hmm, are you sure about that? Keep your answer ready; we will check it in our last example. This is Set three of the mixed JEE Main pattern problems, with three tools today: p K a, the sodium hydrogen carbonate test, and resonance. These come up often in JEE Main, and they are quick marks. So, let's begin.

0:43Tool 1: pKₐ, the ruler of acid strength

So, first tool: p K a. On screen, K a is a capital K with a small a written low, the acid's equilibrium constant, and p K a equals minus log of K a. Because of that minus sign, the smaller the p K a, the stronger the acid. Okay? And one unit of p K a is a factor of ten in strength. So two units apart means a hundred times, and three units, a thousand times. Right? Now let me ask you this. In an acid base reaction, there is an acid on each side. Which side does the equilibrium favour? The side of the weaker acid, yes, exactly! The stronger acid gives away its proton, so we end up with the weaker acid and the weaker base. Clear? And here is the formula. H A plus B minus gives A minus plus H B; the minus signs written high are charges, and the two half arrows mean equilibrium. Then capital K equals ten to the power, p K a of H B, minus p K a of H A; the brackets name the acid, and the raised part is the power. With me? Now a quick one from this set. Ethanol, C two H five O H, reacts with N H two minus, to give ethoxide, C two H five O minus, and ammonia, N H three; the small numbers written low count the atoms. The p K a of ethanol is sixteen, and of ammonia, thirty eight. What is the equilibrium constant? Here H A is ethanol, and H B is ammonia. So the power is thirty eight minus sixteen, which is twenty two, and capital K equals ten to the power twenty two. That is a huge number: ethanol is the stronger acid, so the reaction goes forward. Got it?

2:36Tool 2: which acids fizz with NaHCO₃?

Now, second tool: the order of acidity. R stands for an alkyl group, and the sign between each pair means greater than. So, sulphonic acids, then carboxylic acids, then carbonic acid, H two C O three, then phenol, water, and alcohols. Okay? Now the test. Sodium hydrogen carbonate, N a HCO three, gives off carbon dioxide, the fizz, only with an acid stronger than carbonic acid. And carbonic acid has a p K a of six point four. Clear? So tell me, phenol is an acid. Does it fizz with sodium hydrogen carbonate? No, phenol does not fizz at all! Its p K a is ten, so it is weaker than carbonic acid. Water and alcohols are weaker still, so no fizz there either. Right? But carboxylic and sulphonic acids do fizz. And so do two, four dinitrophenol and picric acid: with their nitro groups, these phenols are as strong as carboxylic acids, or stronger. Got it?

3:42Tool 3: resonance, a lone pair next to C=O

Now, third tool: resonance. When one structure is not enough, we draw two or more with the atoms in the same places; only the pi electrons and lone pairs move. Pi is this Greek letter. With me? Look at ethanamide; the dashes are single bonds, the two lines, a double bond, and the brackets hold the oxygen joined to that carbon. The lone pair on nitrogen sits right next to the carbon oxygen double bond, so it can move in, and the pi electrons move onto the oxygen. The double-headed arrow joins the two structures; it does not mean an equilibrium. On the right, oxygen has a minus charge, and the plus written above the nitrogen is its positive charge. Okay? So tell me, with its lone pair spread out like this, is ethanamide a good base in water? That is right, it is not a good base at all! Amides are not basic in water, because the nitrogen lone pair is delocalised onto the carbonyl oxygen. It is not free to grab a proton. Clear? And remember, the real molecule is a hybrid of these structures, and the hybrid is more stable than any one of them. So losing the resonance costs stability. Keep this in mind for our last example.

4:57Example 1: which of four acids fizz with NaHCO3

Right? Now, example one. Which of these four dissolve in aqueous sodium hydrogen carbonate with effervescence, that is, with fizzing? Phenol, ethanol, four nitrophenol, and ethanoic acid. Pause the video, and try it first. Done? Okay, let's check. Our rule from tool two: an acid fizzes only if it is stronger than carbonic acid, so its p K a must be below six point four. Right? Phenol has a p K a of ten, and ethanol, sixteen. Both are weaker than carbonic acid, so no fizz from these two. Clear? Now four nitrophenol. The nitro group makes it much more acidic than phenol, so it fizzes too, right? Oh no, careful, this is the trap! Its p K a is seven point one, and seven point one is greater than six point four. So it is still weaker than carbonic acid, and there is no fizz. One nitro group is not enough here. Got it? And ethanoic acid has a p K a of four point seven six, below six point four. So it is stronger than carbonic acid, and it fizzes. Okay? So the answer is ethanoic acid only. Really simple, once you compare with six point four. With me?

6:19Example 2: is ethanamide protonated on N or on O?

Now the harder one, JEE level, and it is our opening puzzle. In strong acid, is ethanamide protonated on nitrogen or on oxygen? And why? Here is a hint: think about the resonance. Pause the video and try it first. Done? Alright, first the quick answer. Nitrogen has a lone pair, just like ammonia, so the proton goes to nitrogen, right? No, no, wait, look at what that does! To hold the proton, nitrogen uses its lone pair, and it becomes N H three plus. Now there is no lone pair left to share with the carbon oxygen double bond, so the resonance is lost. And losing resonance costs stability, remember? So now tell me, what if the proton goes to the oxygen instead? Does that cation keep its resonance? Let us see. The oxygen takes the proton, and the plus written above it is its positive charge. But look, the nitrogen still has its lone pair, free to move. Okay? So that lone pair moves in, and the plus charge shifts onto the nitrogen; the double-headed arrow joins the two structures. Yes, it does: the cation keeps its resonance! So the answer is, on oxygen. Protonation on oxygen keeps the resonance, and protonation on nitrogen would destroy it. There you go, puzzle solved: the proton goes where the resonance survives.

7:48Quick revision: pKa rule, NaHCO3 test, amide protonation

Clear? So, quick revision. Smaller p K a means a stronger acid, and capital K equals ten to the power, p K a of H B, minus p K a of H A, okay? Sodium hydrogen carbonate fizzes only with acids stronger than carbonic acid, whose p K a is six point four. And amides are protonated on oxygen, because that keeps the resonance. Now try the rest of Set three on this page, against the clock: twenty five minutes for all fifteen problems. All the best!